Communications devices, particularly those that implement digital subscriber line (DSL) technologies (e.g., T1 and xDSL, including SDSL, HDSL, ADSL, etc.), transmit high speed data using analog signals over telephone connections, which are typically copper wire pairs. The connections and equipment are subject to adverse impulse noise. Impulse noise events are likely correlated over several symbol (or baud) periods of the DSL modulation. Correlated noise or distortion undesirably will significantly degrade performance of the decoder associated with a receiver.
In order to minimize the adverse affects of noise, various forward error correction coding techniques (also known as convolutional coding) have been developed and employed in the past. Typically, in forward error correction coding, at the transmitter, data bits are encoded by adding redundant bits systematically to the data bits so that, normally, only predetermined transitions from one sequential group of bits (corresponding to a symbol, or baud) to another are allowed. There is an inherent correlation between these redundant bits over consecutive bauds. At the receiver, each baud is tentatively decoded and then analyzed based on past history, and the decoded bits are corrected, if necessary.
One well known and widely accepted error coding technique is trellis coded modulation (TCM), which is a form of convolutional coding that is optimized according to a specific modulation scheme. A TCM encoder is situated at the transmitter, and a TCM decoder is situated at the receiver. TCM is highly desirable since it combines the operations of modulation and error coding to provide effective error control coding without sacrificing power and bandwidth efficiency. The TCM decoder essentially averages the noise over more than one of the symbols. However, noise that is correlated over the constraint length of the trellis code will effectively degrade performance. In many cases, correlated noise causes the trellis decoder to perform worse than if the receiver employed no trellis coding at all.
As examples, U.S. Pat. No. 5,659,578 to Alamouti et al. and U.S. Pat. No. 4,677,625 to Betts et al. describe the concept of TCM. The latter describes a distributed trellis encoder that can be used to spread symbols associated with a data stream over time across successive symbol (baud) periods. This distributed encoder significantly improves performance by making the transmissions less susceptible to errors resulting from imposition of correlated noise. U.S. Pat. Nos. 5,659,578 and 4,677,625 are entirely incorporated herein by reference.
The various DSL technologies employ a variety of line coding, e.g. 2 Binary, 1 Quaternary (2B1Q), Quadrature Amplitude and Phase modulation (QAM), Carrierless Amplitude and Phase (CAP) modulation, and Discrete Multitone (DMT). DMT is now the standard line coding for Asymmetrical Digital Subscriber Line (ADSL) as specified in international standards published by the ITU (International Telecommunication Union) as Recommendations G.992.1 Series G: Transmission Systems and Media, Digital Systems and Networks, Digital Transmission Systems—Access Networks ADSL Transceivers, and G.992.2 Splitterless ADSL transceivers. G.992.1 and G.992.2 are available from the ITU, Geneva, Switzerland, at http://vww.itu.int and are entirely incorporated herein by reference.
DMT is a Frequency Division Multiplex (FDM) type of modulation in which an incoming bit stream is multiplexed into a number of sub-carriers or sub-channels. DMT as used in ADSL enables a digital subscriber technology capable of delivering high-speed digital information over existing unshielded twisted pair copper telephone lines DMT encodes data on multiple sub-carriers, referred to as tones, that are then converted to time domain signals for transmission by an Inverse Discrete Fourier Transform (IDFT). An additional level of line coding, e.g. QAM, can be employed within each of the tones. A DMT trellis encoder generally codes between adjacent tones. DMT uses a Discrete Fourier Transform (DFT) to demodulate the tones.
The DMT 16-state trellis code constraint length is approximately four 4-dimensional symbols. Four-dimensional symbols are encoded as two 2-dimensional constellations on two tones. Four 4-dimensional symbols are thus encoded over eight tones. DFT suffers from performance limitations including sinx/x coupling of energy between adjacent tones. DMT convolutional encoders operate “serially” on mapped constellations such that consecutively generated constellations are mapped to adjacent tones. (Sin x)/x coupling allows noise on one tone to affect adjacent tones. Correlated noise on adjacent tones, particularly that within the DMT code constraint length, contributes to multiple metric calculations in the trellis decoder. Correlated noise in consecutive metric calculations causes negative gain and can result in a worse performance than if no coding was employed.
DSL technologies are still in a state of infancy and are being improved over time by engineers and designers. The industry still needs ways to further enhance DSL communications and, in particular, ways to minimize the adverse effects of impulse noise and correlated noise. Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.